Acoustic Vector Probes
Recently a patent application was filed for a new acoustic instrument, the acoustic vector probe (AVP).                1. R. Hickling 2003, “Acoustic Measurement Method and Apparatus”, patent application to the United States Patent and Trademark Office, Ser. No. 10/396,541, Filing Date Mar. 25, 2003.The technical information contained in this application is hereby incorporated herein by reference.        
An AVP consists of a tetrahedral arrangement of four small microphones less than 6 mm in size that simultaneously measures at a point in air the three fundamental quantities of acoustics, namely the sound-intensity and sound-velocity vectors, and sound pressure. Sound intensity is the time average of sound power flow per unit area. The time dependence of sound intensity is determined by taking a series of averages over short intervals. AVPs are more accurate, more compact and less expensive than previous instruments for measuring sound intensity. A calibration procedure described by Hickling (Ref. 1) ensures the probe is accurate and omnidirectional.
The sound-intensity vector determines the direction of a sound source. Because it is expressed as a fast Fourier transform (FFT), it also provides information about the frequency characteristics of the source, enabling the AVP to distinguish one source from another. Sources can also be distinguished by how they occur in time.
The microphones that are used in AVPs can be of the electret type such as the Knowles FG series or the Primo EM123 which respond to ultrasonic frequencies up to about 40 kHz. Also the frequency range of the calibrating microphone, such as the Bruel and Kjaer 4135, extends to about 100 kHz. However measurement with an AVP is presently limited to the audible frequency range below about 15 kHz, because the measurement calculations for the AVP are based on finite-difference approximations that are valid only when the wavelength of sound exceeds the spacing d between microphones, i.e. according to the relation kd<1 where k=2π/wavelength.
Subsequently two continuations-in-part (CIPs) were submitted describing the use of arrays of AVPs                2. R. Hickling, 2003, “Method and Apparatus for Acoustic Detection of Buried Objects”, patent application to the United States Patent and Trademark Office Ser. No. 10/658,076, Filing Date Sep. 9, 2003.        3. R. Hickling, 2003, “Sound Source Location and Quantification using Arrays of Vector Probes”, patent application to the United States Patent and Trademark Office, Ser. No. 10/746,763, Filing Date Dec. 26, 2003.The technical information contained in these CIPs is hereby incorporated herein by reference. They describe how arrays of AVPs can be used for a variety of applications. They also indicate how modern digital signal processing permits simultaneous measurement at all the AVPs in the array.Review of Echolocation and Use of Ultrasonics        
Echolocation (perceiving objects using acoustic echoes) is a well-known concept, particularly for underwater detection and machine perception. The most advanced form of echolocation in air appears to be that of bats whose remarkable abilities have been described by                4. D. R. Griffin, 1958, “Listening in the Dark, The Acoustic Orientation of Bats and Man”, Yale University Press, New Haven.and by        5. J. A. Simmons, 1997, “Bats and Echolocation”, Chapt. 151, 1819–1822, “Encyclopedia of Acoustics”, (M. J. Crocker, Ed.) John Wiley and Sons.Use of echolocation by the blind is discussed by Griffin. A clicking or tapping device is used to generate audible sound pulses and the ears detect the resulting echoes from nearby objects. In general bats use ultrasound which is sound above the frequency range of human hearing. This enables them to detect small objects such as insects and generally to operate at frequencies above background noise, both natural and man-made. The signals emitted by bats have directional characteristics, as described by        6. D. J. Hartley and R. A. Suthers, 1989, “The sound emission pattern of the echolocating bat, Eptesicus fuscus”, J. Acoust. Soc. Amer. 85, 1348–1351.A more easily understood version of the data in this paper is presented by        7. J. A. Simmons, 2002, “Directionality of biosonar broadcasts and reception by the ears”, Tutorial Lecture, Acoustical Society of America, Pittsburgh, Pa., Jun. 2, 2002.The horizontal cross-section of the beam of the emitted signals is regular in shape and generally too wide to distinguish individual objects. For example for frequencies around 35 kHz the beam is roughly 100 degrees wide. Obviously bats cannot distinguish individual objects with so wide a beam. Instead they have to depend on their hearing system to achieve the resolution needed for echolocation.        
Ultrasound is used in devices, such as range finders in cameras, distance measuring systems and depth gages. Generally the same transducer is both the source and receiver. The transducer emits a pulse and waits to receive the echo before emitting another pulse. Perhaps the most widely known system of this kind is manufactured by the Polaroid Corporation of Wayland, Mass. for range finding by a camera. This has been used extensively in research studies, for example by                8. D. Lee, 1996, “The Map-Building and Exploration Strategies of a Simple Sonar Equipped Mobile Robot” Cambridge University Press.This book illustrates signal processing methods associated with use of the Polaroid sensor. In addition ultrasonic sensing systems for industry are manufactured by The Ultrasonic Arrays Company of Woodinville, Wash. where the source and receiver are again generally the same transducer.        
The use of the same transducer as a source and receiver has disadvantages. There is a time constraint because the transducer has to wait to receive the echo before emitting another signal. Also it can only receive echoes from ahead of the transducer. With the echolocation system of bats, on the other hand, the source and receiver are separate. A bat can emit ultrasonic signals whenever it wants and can hear echoes and other sounds from many directions. In this way it is able to echolocate successfully.
It is not easy for humans, whose dominant sense is vision, to relate to echolocation. Vision operates passively, because objects are perceived only when illuminated by natural or artificial light. Echolocation, on the other hand, is active, objects being perceived when sound emitted by a source operates in conjunction with a receiver. Also there is a major difference between reflected light and reflected sound. Usually light is scattered in all directions from all points on a surface, so that every point on the surface can be seen by the eye. On the other hand, sound is usually backscattered by mirror-like highlights where these highlights are the only parts of a surface that can be perceived at any one time. A complex surface generally will have more highlights and will reveal more of itself. Additional information is obtained when the source/receiver and the reflecting surface are in relative motion, so that the highlights move over the surface. The bat's motion through the air is therefore a major part of it's ability to echolocate. To return a detectable echo, an object must be larger than the wavelength of the incident sound. The strength of an echo is generally determined by the radius of curvature at the location of a highlight on a surface.
Distance to a reflecting object is determined by the round-trip time of flight of an acoustic signal, i.e. the time taken for sound to travel from the source to the object and back to the receiver. Multiplying the time of flight by the speed of sound and dividing by two gives the distance. Two methods have been used to measure time of flight. The most common method uses sound pulses or bursts of sound. Time of flight is the interval of time between the departure of the outgoing pulse from the source and the return of the corresponding echo to the receiver. A feature of the pulse, such as the leading edge or its maximum amplitude, is used as a time marker. This method is used in devices, such as range finders in cameras, and depth gages.
The second, less common method uses frequency modulation. Here the outgoing sound generally consists of a continuous signal with a saw-tooth frequency modulation, whose frequency sweep is related to the distance between the source and the reflecting object. Because the source transmits a continuous signal, a separate receiver is required. The echoes have a corresponding saw-tooth frequency modulation, delayed relative to the outgoing signal by the round-trip travel of the sound. The received signal is then heterodyned or mixed with the outgoing signal. This generates a trace of pressure amplitude versus time, or time response (based on frequency differences), which determines the distances of various reflecting surfaces from the source/receiver. The method has been is described by                9. R. C. Heyser, “Acoustical Measurements by Time Delay Spectrometry” U.S. Pat. No. 4,279,019, July, 1981.A similar method was developed as an aid to the blind by        10. L. Kay, 2000, “Auditory perception of objects by blind persons, using a bioacoustic high resolution air sonar”, Journ. Acoust. Soc. Amer., 107(6), 3266–3276.The device has earphones and is worn on the head.        
Bats use frequency-modulated pulses, the frequency generally decreasing from the beginning to the end of the pulse. Distance to an object is determined by the time of flight of the pulses. Frequency modulation of the echoes compared to the frequency modulation of the outgoing pulsed signals provides additional information. Echoes using frequency modulated pulses were studied by                11. R. Hickling and R. W. Means, 1968, “Scattering of Frequency-Modulated Pulses by Spherical Elastic Shells in Water,” Journ Acoust. Soc. Amer., 44, 5, 1246–1252.        
Incident sound can generate a vibrational response in an object, as shown, for example, by                12. R. Hickling, 1962, “Analysis of echoes from a solid elastic sphere in water”, Journ. Acoust. Soc. Amer., 34, 1582–1592.This gives the echo a quality determined by the internal structure of the reflecting object and is probably used by bats. The effect is relatively weak for solid objects in air, compared to solid objects in water.Bat Detectors        
The ultrasonic signals of bats can be changed to audible frequencies by using an electronic process called heterodyning or mixing. This is a standard procedure in radio technology, as described for example in                13. D. B. Rutledge, 1999, “The Electronics of Radio”, Cambridge University Press.Griffin was the first to apply heterodyning to bat signals to make them audible to the human ear and the method has been used extensively since then.        